1. Field of the Invention
This invention relates to a semiconductor device and particularly, but not exclusively, to a semiconductor laser device that emits visible radiation in the wave-length range 630 nm to 680 nm. The laser device may be of the edge-emitting or of the surface-emitting type.
2. Description of the Related Art
Laser devices or laser diodes (LDs) fabricated in the (Al,Ga,In)P material system which emit visible light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer products. For example, it is envisaged that the Digital Video Disc (DVD) system will employ a 635 nm-650 nm wavelength LD capable of delivering up to 30 mW output power up to a temperature of 60xc2x0 C. The next generation of semiconductor lasers will need an even greater maximum power output up to a higher (eg. 70xc2x0 C.) operating temperature.
By the (Al,Ga,In)P system is meant the family of compounds having the general formula (AlxGa1xe2x88x92x)1xe2x88x92yInyP, where both x and y are between 0 and 1. One particular advantage of this semiconductor system is that it is lattice-matched to a Gabs substrate when the indium mole fraction, y, is equal to 0.48.
A principal limitation of current (Al,Ga,In)P laser diodes is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperature. It is generally believed that this is caused by electron leakage from the active region of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
The generic structure of a separate confinement laser structure intended to generate light at 630-680 nm will now be described with reference to FIGS. 1(a) and 1(b).
FIG. 1(a) is a schematic band structure of a separate confinement laser structure fabricated in the (Al,Ga, In)P system. It consists of an n-doped (Al0.7Ga0.3) 0.52In0.48P cladding region 1, an (Al0.5Ga0.5)0.52In0.48P optical guiding region 2, 4, a GaInP quantum well active region 3 disposed within the (Al0.5Ga0.5)0.52In0.48P optical guiding region, and a p-doped (Al0.7Ga0.3)0.52In0.48P cladding regions. A p-type contact layer (not shown in FIG. 1(a)) may be provided on the p-type cladding region 5, and an n-type contact layer (also not shown in FIG. 1(a)) may be provided on the n-type cladding region 1. Optical transitions giving rise to laser action in the quantum well active region 3 of the laser diode originate from electrons in the xcex93-band in the GaInP quantum well active region.
The terms xcex93-band and X-band as used herein refer to symmetry points it the Brillouin zone and are standard terms in solid state physics, see for example R. A. Smith xe2x80x9cSemiconductorsxe2x80x9d, (Cambridge University Press, 1978). The terms xcex93-minimum and X-minimum refer to the minimum energy level of the xcex93-band and the X-band, respectively.
The minimum energy in the conduction band of (Al,Ga,In)P is a function of the aluminium content. There is a crossover from a xcex93-band minimum to an X-band minimum at an aluminium concentration of about 0.55.
FIG. 1(b) illustrates a similar laser structure to that of FIG. 1(a), but the cladding regions 1,5 are formed of AlInP rather than (Al0.7Ga0.3)0.52In0.48P in order to increase the potential barrier confining xcex93-electrons in the optical guiding region 2.4.
In FIGS. 1(a) and 1(b) the active region 3 is shown as containing a single quantum well layer. As is well known, however, the active region 3 may alternatively consist of a plurality of quantum well layers, with each quantum well layer being separated from an adjacent quantum well layer by a barrier layer.
Degradation of semiconductor lasers has been a major problem in developing commercial devices. AlGaAs/GaAs lasers having a lasing wavelength of around 0.85 xcexcm were initially developed in the 1970s, but early examples of these lasers degraded quickly during use and, as a result, had a low lifetime and were unsuitable for commercial applications. It took a considerable time to overcome the significant degradation problems involved with these lasers. One of the main factors in the degradation of AlGaAs/GaAs lasers was crystal defects introduced during the crystal growth and fabrication processes These defects are known as xe2x80x9cdark linesxe2x80x9d defects, and give rise to a localised region of high non-radiative recombination centres which have high optical absorption. The problem of these dark line defects was eventually overcome by improving the structural quality of the GaAs, and long lifetime AlGaAs,GaAs lasers are now commercially available. M. Fukuda reports, in xe2x80x9cReliability and Degradation of Semiconductor Lasers and LEDsxe2x80x9d ISBN 0-89006-465-2, that AlGaAs/GaAs lasers having a life time greater than 10,000 hours are now commercially available.
In order for (Al,Ga,In)P lasers to be commercially successful, these lasers must have a life time comparable with that of AlGaAs/GaAs lasers.
At present, wide bandgap phosphide lasers operating in the visible spectrum at a wavelength of about 650 nm display a severe degradation problem. Although the lifetime of low power phosphide lasers is approximately 10,000 hours which is satisfactory for commercial purposes, a typical lifetime of a high-power phosphide laser is only about 5,000 hours which is not commercially acceptable. Furthermore, it is necessary to anneal the lasers in order to obtain these lifetimes, and lasers that are not annealed have much shorter lifetimes.
The degradation problem is particularly serious for lasers fabricated using molecular beam epitaxy (MBE). At present, phosphide laser structures that are grown by MBE have to be thermally annealed in order to improve their reliability and to decrease the threshold for laser operation. It is presumed that the annealing process removes (or at least moves) some of the non-radiative recombination centres in the material. It is, however, undesirable to carry out an annealing step. One common p-type dopant for the p-type cladding region is beryllium, and if a beryllium-doped laser device is annealed beryllium can diffuse from the p-type cladding region into the active region. Such diffusion will degrade the performance of the laser device, and may also lower the yield of the manufacturing process.
M. Jalonen et al., report, in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 71 No. 4 p.479 (1997), laser oscillation thresholds for lasers grown by the MOCVD process in the range 0.2-0.4 kA/cm2 compared to thresholds for material grown by MBE of  greater than 1 kA/cm2, for an emission wavelength of 680 nm. It is possible that the poor degradation characteristics and higher laser thresholds for lasers grown by MBE arise because these materials have significantly more non-radiative recombination centres (or defects) than material grown by MOCVD. It 1s desirable to remove non-radiative defects in order to improve the reliability of phosphide lasers, and to lower their lasing threshold.
One particular type of defect that could well be responsible for the severe degradation and high laser thresholds in wide bandgap phosphide lasers is the anti-phase domain defect (APD). These defects arise in phosphide materials as a consequence of ordering occuring in the crystal structure. APDs give rise to a region of high non-radiative recombination, and of high optical absorption. Thus, the higher is the density of APDs, the higher will be the optical absorption and the higher will be the threshold for laser oscillation. A high density of domain boundaries also results in fluctuations in the refractive index of the material, and these fluctuations will cause additional photon loss and will hence decrease the quantum efficiency of the laser. The presence of APDs may also lower the yield of the manufacturing process.
C. Geng et al report, in xe2x80x9cJournal of Crystal Growthxe2x80x9d Vol. 170, page 418 (1997), that the domain structure can be responsible for the worsened laser performance of an ordered structure.
A. Valster et al. report, in xe2x80x9cJournal of Crystal Growthxe2x80x9d Vol. 107, page 403, (1991), that ordering must be avoided in order to produce a laser having a low threshold for laser oscillation.
T-Y Seong et al. report, in xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol. 70, No. 23, page 3137 (1997), that APDs decrease the carrier mobility, Introduce deep energy levels, and decrease carrier life times.
APDs start to form after about 1 xcexcm of epitaxial growth, although this depends on the growth parameters such as the growth temperature and the V/III ratio during the growth process. In an (Al,Ga,In)P laser having the general structure shown in FIGS. 1(a) or 1(b), APDs usually form in the n-type contact layer (if one is provided) or in the n-type cladding layer 1, and extend right across the semiconductor laser through the active region to the p-type contact layer 6 disposed on the p-type cladding layer 5. FIG. 2 is a schematic sectional view of an SCH laser, such as the laser of FIG. 1(a) or FIG. 1(b) fabricated in the (Al,Ga,In)P system. APDs 7 run through the laser structure, from the n-type cladding region 1 to the p-type contact region 6.
TEM photographs of APDs running through laser structures are shown by C. Geng et al (above).
EP-A-0 603 780, EP-A-0 421 205, U.S. Pat. No. 5,508,554 and U.S. Pat. No. 4,835,166 relate to the growth of a semiconductor layer structure on a substrate that is not lattice-matched to the layer structure. The lattice mis-match between the substrate and the layer structure lead to the formation of defects and dislocations, and these documents propose growing a buffer layer at the interface between the substrate and the layer structure to reduce the effects of the lattice mis-match. These documents do not, however, address the problem of the formation and propagation of defects within the layer structure itself.
A. Valster et al (supra) report that the size of domains caused by ordering can vary by as much as a factor of 50, depending on the growth temperature and the aluminium mole fraction of the layer. They report domain sizes of up to 100 s of nm in material grown by MOCVD. T-Y Seong et al (supra) report a smaller domain size for material grown by gas source MBE, of typically 1.5 nm. A large number of domain boundaries will cause additional optical loss, and thus decrease the quantum efficiency of the laser device.
M. Jalonen et al (supra) propose that the threshold for laser oscillation of structures grown by MBE can be reduced significantly by annealing the laser structure, in order to remove non-radiative recombination defects. This is not, however, entirely satisfactory since, as noted above, annealing a device that contains beryllium as a dopant can cause diffusion of the beryllium.
It has been found by C. Geng et al (above) that a high concentration of zinc as a p-type dopant will suppress ordering in the p-type cladding region, but not in the active region.
TEM (Transmission Electron-Micrograph) studies of GaInP layers have shown that a large change in the V/III ratio can also suppress the propagation of APDs in GaInP. This is because the domain size depends, among other factors, on the V/III ratio. Thus, changing the V/III ratio during the growth process will change the domain size and will suppress the propagation of APDs. A large change in the temperature during the growth process will also cause a change in the domain size and thus suppress propagation of APDs. The disadvantage of changing the V/III ratio or the temperature during the growth process in order to suppress propagation of APDs is that an ordered/disordered interface is set up. This gives rise to a difference in bandgap, and so increases the resistivity of the layer.
P. L. Gourley et al report, in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 47 No. 5 page 482 (1985), reported the elimination of dark line defects in the AlGaAs/GaAs material system by using strained layer superlattices, such as GaP/GaAsP or GaAs/GaAsP superlattices. Furthermore, U.S. Pat. No. 5,300,794 discloses growing a strained superlattice on the buffer layer in order to suppress upwardly propagating dislocation defects in,a GaAs/GaAlAs laser structure. The disadvantage of these approaches is that using such superlattices will create a quantum well and/or barrier at each interface, and this will give rise to a high resistivity region. The GaAs/GaAlAs superlattice proposed in U.S. Pat. No. 5,300,794 will impede carrier injection into the active region of the laser device, thus increasing the resistance of the device.
The present invention provides a semiconductor device comprising: a substrate; and a multilayer structure disposed on the substrate, the multilayer structure comprising a first semiconductor layer and a second semiconductor layer disposed on the first semiconductor layer; characterised in that the device further comprises a layer for inhibiting the propagation of defects from the first semiconductor layer into the second semiconductor layer.
If defects such as APDs propagate in the first semiconductor layer, their propagation will be inhibited at the layer for inhibiting the propagation of defects, A reduced number of APDs or other defects will reach the second semiconductor layer, and this will reduce the density of absorption defects in the second semiconductor layer, thus improving the characteristics of the layer.
The second semiconductor layer may be an active layer of the device. The first semiconductor layer may be a cladding layer of the device. The device may be a separate confinement heterostructure laser device having an optical guiding region disposed on the cladding layer, the active layer being disposed in said optical guiding region. The present invention can be applied to a laser device, for example an SCH laser device, to improve the optical characteristics of the active layer of the laser device by inhibiting the propagation of defects such as APDs into the active layer.
The device may comprise the cladding layer, the optical guiding region and the active layer in this order. The layer for inhibiting the propagation of defects may be disposed within the cladding layer. Alternatively, the layer for inhibiting the propagation of defects may be disposed at the interface between the cladding layer and the optical guiding region, or it may be disposed within the optical guiding region.
The device may be an (Al,Ga,In)P device, and the layer for inhibiting the propagation of defects may be a Ga1xe2x88x92yInyP layer. This allows the characteristics of device fabricated in the (Al,Ga,In)P system, such as an (Al,Ga,In)P laser device, to be improved. There is no need to anneal the device, so that the problem of diffusion of dopants during the annealing is eliminated. The layer for inhibiting the propagation of defects may be a strained Ga1xe2x88x92yInyP layer.
The layer for inhibiting the propagation of defects may be disposed within 1 xcexcm of the active layer. The typical domain size for APDs in a phosphide material is around 1 xcexcm, so placing the layer for inhibiting the propagation of defects within 1 xcexcm of the active layer means that APDs cannot re-form in the layers between the active layer and the layer for inhibiting the propagation of defects.
The bandgap of the layer for inhibiting the propagation of defects may be substantially equal to the bandgap of a semiconductor layer adjacent to the layer for inhibiting the propagation of defects. This prevents the formation of a significant potential well or potential barrier, thus ensuring that the transport of carriers through the device is unaffected by the provision of the layer for inhibiting the propagation of defects.
The layer for inhibiting the propagation of defects may be disposed within the first semiconductor layer, and the bandgap of the layer for inhibiting the propagation of defects may be substantially equal to the bandgap of the first semiconductor layer. Alternatively, the layer for inhibiting the propagation of defects may be disposed within the optical guiding region, and the bandgap of the layer for inhibiting the propagation of defects may be substantially equal to the bandgap of the optical guiding region. Alternatively, the layer for inhibiting the propagation of defects may be disposed at the interface between the cladding layer and the optical guiding region, and the bandgap of the layer for inhibiting the propagation of defects may be substantially equal either to the bandgap of the optical guiding region or to the bandgap of the cladding layer.
The first semiconductor layer may be an (AlyGa1xe2x88x92y)0.52In0.48P layer (0.5 less than y less than 1). The first semiconductor layer may be an (Al0.7Ga0.3)0.52In0.48P layer and the layer for inhibiting the propagation of defects may be a Ga0.3In0.7P layer. Alternatively, the first semiconductor layer may be an Al0.52In0.48P layer and the layer for inhibiting the propagation of defects may be a GaP layer. The optical guiding region may comprise an (AlyGa1xe2x88x92y)0.52In0.48P layer (0.3 less than y less than 0.5). The optical guiding region may comprise an (Al0.5Ga0.5)0.52In0.48P layer and the layer for inhibiting the propagation of defects may be a Ga0.38In0.62P layer. For each of these pairs of materials the bandgaps of the two materials are substantially equal to one another, so that provision of the layer for inhibiting the propagation of defects does not cause a significant potential barrier or potential well. The cladding layers and the optical guiding region of an (Al,Ga,In)P laser device are often formed of (Al0.7Ga0.3)0.52In0.48P and (Al0.5Ga0.5)0.52In0.48P respectively, so that the invention allows a layer for inhibiting the propagation of defects to be incorporated into an (Al,Ga,In)P laser device without affecting the transport of carriers through the laser device.
The device may further comprise a second layer for inhibiting the propagation of defects into the second semiconductor layer.
The or each layer for inhibiting the propagation of defects may be for inhibiting the propagation of anti-phase domain defects into the second semiconductor layer.
The characteristic size of an anti-phase domain defeat in the or each layer for inhibiting the propagation of defects may be different from the characteristic size of an anti-phase domain defect in the second semiconductor layer. This is a straightforward way of inhibiting the propagation of anti-phase domains.